The psychological trauma of fierce combat in Iraq may have helped drive soldiers in a single battle-scarred Army unit to kill as many as 11 people after their return home, the military said Wednesday.

In a report billed as the most comprehensive examination to date of violent crimes and combat exposure, an Army task force of medical experts looked at members of Fort Carson's 4th Brigade Combat Team, 4th Infantry Division, known as the Iron Eagles.

Soldiers in the unit were accused in five killings in separate attacks around Colorado Springs in 2007 and 2008, including the slaying of a couple gunned down while posting a garage-sale sign. They were also involved in six more slayings in Colorado and other states since 2005.

The report compared the unit of about 3,700 soldiers with a similarly sized unit and found it suffered more combat deaths in Iraq and was deployed there longer.

"This investigation suggests a possible association between increasing levels of combat exposure and risk for negative behavioral outcomes," the study said. It added that the soldiers faced "significant disruptions in family-social support."

During its two long tours in Iraq, the unit suffered 113 combat deaths, according to Fort Carson spokeswoman Brandy Gill. The unit is now in Afghanistan, where it has suffered two combat deaths since arriving in May.

Lt. Gen. Eric B. Schoomaker, the Army's surgeon general, said the unit's crime cluster appeared to be unique among Army bases and its combat exposure and length of deployments are just two factor officials are looking at.

The soldiers accused of the killings had committed crimes before and abused drugs and alcohol. Also, some of the GIs claimed they were discouraged from seeking mental help, or were too afraid of being belittled if they did so, officials said.

Task force members suggested the Army find a way to identify soldiers who have been exposed to fierce combat. They also recommended better training for officers to manage soldiers with behavioral problems and ensure that GIs who seek help are not humiliated.

"Identifying at-risk soldiers is a complex issue. How do we know which soldier may be the very one to take his own life or the life of someone else?" said Maj. Gen. Mark Graham, commanding general, Division West, 1st Army.

Nationally, at least 121 Iraq and Afghanistan veterans have committed a killing in the U.S. or been charged in one. Between 2004 and 2008, 2,726 Army soldiers were involved in violent crimes out of a force of 1.1 million, said Lt. Gen. Michael D. Rochelle, the Army's deputy chief of staff.

Investigators focused on 14 soldiers accused of murder, manslaughter, attempted murder and aggravated assault, mostly with guns. Two of those 14 were not sent to Iraq.

Among the 12 who were, investigators found that they experienced heavy combat in Iraq and that half of those interviewed reported witnessing war crimes, including the killing of civilians. Schoomaker stressed that the Army could not substantiate the reports of war crimes.

Back home, the soldiers carried weapons with them because they felt "naked" and unsafe and had difficulty transitioning to civilian life. Some said they felt "weird" and didn't fit in, the Army report said.

"There, we were the law; here, the cops are the law," one of the accused told investigators.

The rate of 4th Brigade Combat Team members killed in combat, wounded or otherwise unable to fight was 8.9 per 1,000 soldiers during its first Iraq tour and 9.6 per 1,000 on its second. The other unit, the Fort Carson-based 3rd Brigade Combat Team, had rates of 0.4 and 2.1 per 1,000, respectively.

The study comes as the Army struggles with other combat-related issues, including increased rates of post-traumatic stress syndrome and soldier suicides.

A study last year by the RAND Corp. think tank estimated that nearly 20 percent of returning veterans, or 300,000 people, have symptoms of PTSD or major depression.

Army suicides have increased yearly since 2004 as soldiers deal with longer and repeated tours in Iraq and Afghanistan. Eleven soldiers at Fort Campbell, Ky., have killed themselves this year, and the Army has made suicide prevention training mandatory.

The Colorado slayings include the 2008 deaths of a man and a woman killed by a man with an AK-47 assault rifle as they put up garage-sale signs on a street. Pfc. Jomar Dionisio Falu-Vives is charged with first-degree murder. He told friends he fired on the couple while driving and liked hearing the sirens as authorities raced to the scene, according to police.

In May, Thomas Woolly, a Fort Carson soldier and Purple Heart recipient, was charged with manslaughter in the slaying of a 19-year-old woman. Woolly was in Fort Carson's Warrior Transition Unit, for soldiers who were injured or have psychiatric disorders.

In October, Spc. Robert Hull Marko led investigators to the body of 19-year-old Judilianna Lawrence, whom he met on the Web site MySpace. Police said Marko told investigators he had violent sex with Lawrence before slitting her throat and leaving her to die in the foothills west of Colorado Springs.

Marko also is charged with sexually assaulting a 14-year-old girl five days before Lawrence was killed.